1. Field of the Invention
The invention relates to a method and apparatus for performing electrical power conversion. More particularly, the present invention relates to a method and apparatus for resynchronizing a power converter such as an inverter without using a rotor position sensor.
2. Description of the Related Art
A conventional synchronous motor typically uses rotor position sensors to provide information regarding the position of the motor's rotor with respect to the motor's stator windings to achieve control of the speed and torque. Rotor position sensors such as Hall effect devices are typically mounted in the stator, proximate the stator windings. The rotor position sensors provide rotor position information, which allows for proper control of the conversion of power that is supplied to the stator windings.
However, rotor position sensors can be unreliable due to mechanical alignment problems (e.g., problems caused by bearings) and temperature incompatibility problems between the stator windings and electronic components such as the Hall effect devices. Moreover, the rotor position sensors can be difficult to mount to the motor during motor assembly, especially for multi-pole motors. In multi-pole motors, the electrical misalignment angle is equivalent to the angular mechanical misalignment angle multiplied by the number of pairs of poles. Eliminating the rotor position sensor and its associated interface circuitry and cable to the controller can lower the weight of the overall system, but also increase the robustness of such systems, which is very desirable in aerospace applications where the size and weight and reliability are premium characteristics of the systems offered.
In response to the problems with rotor sensors, sensorless control techniques have been developed for synchronous machines. Sensorless control techniques allow the control of the machine without the use of physical rotor sensors.
In such a sensorless control system, if there is a power interrupt to the controller, the synchronization with the synchronous motor can be lost. When the power comes back on and the rotor is still spinning, if the inverter were to try to synchronize with the synchronous machine to initiate the drive, a current value exceeding the rated current can flow through the synchronous machine causing trauma to the electronic components of the controller. For example, power semiconductor switches such as IGBTs can be damaged. Without any resynchronization scheme the stopping time of the rotor can vary due to the inertia of the system. For example, for large drives this can take tens of seconds. Therefore, it would require long waiting times to be introduced to the controller's logic for a restart. Thus, conventional sensorless control techniques would have to wait for the synchronous machine to come to a full stop before start-up of the drive can begin. This can be quite important, for example, in critical drive systems in aerospace applications. For example, an electrical drive system should engage in the shortest amount of time possible to achieve resynchronization without waiting for the rotor to come to a full stop.
Thus, there is a need for a sensorless control drive technique without using a rotor position sensor that does not have to wait for the synchronous machine to come to a full stop before synchronizing with the synchronous machine in order to initiate the drive. This will allow for a controlled re-initiation of torque for motoring and power for a generating mode of operation.